Light sources for endoscopic use are generally of two types: incandescent filament lamps and arc lamps. Incandescent lamps produce light by passing current through a tungsten filament, causing it to radiate light in proportion to its blackbody color temperature. The hotter the filament, the higher its color temperature and the more nearly it approaches daylight with a color temperature of approximately 5500K. Tungsten filament lamps range in color temperature from approximately 2400–3400K. Because of the low color temperature, objects illuminated by a tungsten filament light source appear slightly yellow due to the low output of blue light from these sources. Arc lamps produce light by creating a plasma between two electrodes within the sealed bulb. White light from these lamps can be produced by choosing the appropriate fill gas (usually Xe) and pressure (usually several atmospheres). Color temperature of common arc lamps is approximately 4000–6000K. Both types of lamps, filament and arc, are very inefficient in converting electrical power to light, and consequently produce large amounts of heat. The heat must be dissipated. It also contributes to a shortened useful life of such light sources.
There have been numerous attempts to utilize low power (<1 W electrical power consumption, typically operating below 100 mW) LEDs coupled to fiber optic light guides as light sources for endoscopy, dentistry, and for remote illumination of objects (as with a flashlight). Most of these prior attempts employ numerous low power LEDs for remote illumination. Multiple LEDs are necessary because the light output from a single, low power LED is very low and there is poor coupling of light emitted by the LED(s) into the optical fiber. An example of several coupling methods appears in U.S. Pat. No. 6,331,156 whereby the inventors place the fiber optic directly in front of either a surface mount or cylindrical LED without the use of additional optical components, coatings or gel. This patent also exemplifies the use of additional optical components in the form of lenses or mirrors in order to collect light generated from a standard, unmodified LED package configuration. US published patent application 2004/0004846 A1 utilizes a lens to couple the light emitted by an LED into a fiber optic. US published patent application 2003/0156430 A1 describes a device that consists of a number of individual LEDs mounted on mirrors, the arrangement of the LEDs and mirrors having a common focal point at the input end of the fiber light guide. U.S. Pat. No. 6,260,994 describes a plurality of LEDs mounted between a spherical or parabolic reflector and a lens, which directs the light emitted by the LEDs into the light guide fiber. In this invention, the LEDs emit light towards the collecting lens and away from the reflector. In U.S. Pat. No. 6,318,887, the LEDs are positioned so as to emit radiation towards a reflector, which then reflects light through a transparent printed circuit board and towards a lens and fiber light guide. In US published patent application 2002/0120181 A1, light emitted by several LEDs is collected along a common optical axis through a series of beam splitting prisms, in at least one embodiment with the use of lenses to couple the light from the LED into the prism, and then into the fiber. In these examples, the LED packages are not modified; multiple, low power LEDs are employed in order to attain a reasonable level of illumination; and in most of these examples, external optical components are employed in order to increase the coupling efficiency between the LEDs and the light guide fiber.
In US published patent application 2003/0231843, numerous low power LEDs are coupled into individual fibers, which are combined together at the distal end of the device to produce intense light for curing dental epoxy. This patent application describes an approach in which the LED package and light guide fiber are modified in order to increase the optical coupling efficiency between the two. In one embodiment, the cladding material from the fiber is removed and the fiber core is placed within the LED epoxy lens. The exterior of the epoxy lens is coated with a low refractive index “clad” that produces total internal reflection of the light emitted by the LED. Some of the light reflected by this LED clad can make its way into the core of the fiber and be transmitted to the distal end of the device. This patent application also includes a description of a taper attached to the external surface of the LED dome lens that couples the light into the fiber. Again, however, additional optical elements (cladding or tapers applied to the LED) are used. Also, the device employs numerous low power LEDs to attain sufficient light output from the device. This type of arrangement would be difficult if not impossible to implement with a high power LED because of the high operating temperature of these LEDs (up to 135 degrees C.). At high temperatures, the epoxy used in typical LED packages will melt or crack due to thermal cycling. In addition, high temperatures will cause the epoxy to discolor, typically becoming yellow. This will impart a yellow cast to the light, thereby lowering its effective color temperature and its desirability as a visual light source. In addition, discoloration will absorb lower wavelengths of light emitted by the LED, particularly those wavelengths in the blue and UV region of the spectrum that are essential for epoxy curing and fluorescence applications.